This invention relates to water vapor treated metal promoted zeolites. In a more specific aspect it relates to water vapor treated zinc promoted zeolite/alumina compositions. In another aspect it relates to a process for converting non-aromatic hydrocarbons to lower olefin and aromatic hydrocarbons in the presence of a water vapor-treated metal promoted zeolite
It is known to catalytically crack non-aromatic gasoline boiling range hydrocarbons to lower olefins (such as ethylene and propylene) and aromatic hydrocarbons (such as benzene, toluene, and xylenes i.e. BTX) in the presence of zeolite catalysts such as ZSM-5, as is described in an article by N.Y. Chen et al in Industrial & Engineering Chemistry Process Design and Development, Volume 25, 1986, pages 151-155. The reaction product of this catalytic cracking process contains a multitude of hydrocarbons such as unconverted C5+ alkanes, lower alkanes (methane, ethane, propane), lower alkenes (ethylene and propylene), C6-C8 aromatic hydrocarbons (benzene, toluene, xylenes, and ethylbenzene), and C9+ aromatic hydrocarbons. Depending upon the relative market prices of the individual reaction products, it can be desirable to increase the yield of certain of the more valuable products relative to the others. Generally it is desirable to enhance the yield of ethylene, propylene and BTX.
Many zeolite catalyst modification techniques are known to enhance one characteristic or another of the catalyst, but they generally tend to cause an undesirable deterioration in other desired characteristics such as selectivity or catalyst life.
Zinc promoted zeolite catalysts for conversion of lower value hydrocarbons to higher value materials such as lower olefins and BTX are well known. Such catalysts, however, deteriorate rapidly during use due to zinc loss and coke formation. A zinc promoted catalyst resistant to zinc loss as a result of a water vapor treatment in the presence of alumina is disclosed in Japanese patent 7-29948, published Apr. 5, 1995. Such catalysts are commercially available from Asahi Chemical Industry Co. under the designation "ALPHA" catalyst.
Coke formation during zeolite catalyzed aromatization of hydrocarbons has continued to be a problem, however.